Evidence for Gloeocapsomorpha prisca in Late Devonian source rocks from Southern Alberta, Canada

Abstract

Organic-rich intervals within the Late Devonian Camrose Member of southern Alberta, Canada show optical and chemical characteristics very similar to those of Ordovician samples whose organic matter is dominated by Gloeocapsomorpha prisca-derived alginite. This includes saturate fraction gas chromatograms dominated by n-alkanes with a pronounced odd carbon number up to C19, and much lower abundances of C19+n-alkanes, acyclic isoprenoids and biomarkers. Microscopy reveals these samples contain G. prisca-like alginite with a morphology that previously has been suggested for the growth of this organism under higher salinity conditions which is in agreement with the depositional environment of the Camrose Member. One unusual feature of the G. prisca-rich samples is the very high concentration of C21 and C23n-alkylnaphthalenes and n-alkylbenzenes in the aromatic fractions of their extracts. The origin of these compounds is not known but their occurrence could be related to the high concentrations of C21 and C23n-alkyl phenols that have been noted in pyrolysates of Estonian kukersites by previous workers. Other Camrose Member extracts show very different distributions that are more typical for carbonates deposited under anoxic, higher salinity conditions. Many oils that occur in Nisku Formation reservoirs in the Enchant-Hays field in southern Alberta show saturate fraction gas chromatograms and other evidence that indicate an important contribution to these oils from the G. prisca-rich intervals. These are thought to have a contribution from Camrose Member source rocks. Hence, caution should be used in only attributing G. prisca geochemical characteristics to Cambro-Ordovician oils as, on rare occasions, they can also occur in younger source rocks and their derived oils.